How To Convert PDF Document To Images and Vice Versa

To Convert PDF to JPG:
convert some.pdf some.jpg
# this will create some.jpg.PAGE# for each page in the PDF

Alternately if you want custom naming scheme:
convert some.pdf some%03d.png
# this will create somePAGE#.png

To create PDF out of multiple image in a directory:
cd img_dir && convert *jpg some.pdf
#places each image on it's own PAGE

If you need more info than this don't stop Googling.